Manually controlled movement of a robot arm is used especially during a programming stage in order to teach the robot to follow an operating path defined by a number of set points. A memory means in the control system may store the coordinates of said set points as position instructions and other operating instructions which, together with said set points, may define a work cycle of the robot and means arranged, during automatic running of the robot, to control the robot in accordance with information stored in the memory means during the programming stage.
In a control system for an industrial robot, the robot during programming is often run with the aid of a first control means which can be manually influenced, a means such as a joystick. The joystick is often comprised in a portable control unit also called a Teach Pendant Unit or TPU. The robot or end effector of the robot is successively moved to each of said set points or waypoints which define the movement path or the operating positions that the robot is to follow or assume during automatic operation. These positions may be stored in the form of a sequence of position instructions in a program memory, i.e. the memory means, present in the robot, or in a control unit. In addition to the pure position instructions, other operating instructions are stored in the program memory giving further information about the working cycle of the robot. For example the operating instructions may give information about the desired robot velocity in a certain section of the movement, about control of outputs from the robot to external equipment, about a desired precision of the robot movement, about conditions for the robot movement in dependence on input signals from external sensors or equipments, about events such as switch on/off paint spray, about calls for pre-programmed subroutines, and so on. During automatic operation of the robot, the program instructions are run through successively, and the robot is displaced successively between the different programmed positions in accordance with the position instructions and further carries out the measures which are programmed into the instructions.
U.S. Pat. No. 4,888,708 to Brantmark entitled Control system for an industrial robot describes a portable unit or teach pendant unit for manual control of the robot during programming for teaching the robot to follow a trajectory or working path defined by a number of set points. The TPU described comprises a joystick arranged for controlling the movement of the robot as well as other control means with a plurality of other control functions.
A control unit or TPU for use with robot must include a safety switch of the type known as a dead man's handle or also known as a live handle switch. This device requires the operator to hold a switch in a run position such that if the operator lets go of the switch, the machine stops. A live handle switch of a TPU may comprise a certified safety switch type as described in U.S. Pat. No. 5,444,203 to Gunnarsson assigned to ABB entitled Safety Switch for an Industrial Machine. The above specification describes a three position safety switch, which is operable from a first end position via an operating position to a second end position, the safety circuit being closed (made) only in the operating position.
The present invention aims to provide an improved safety switch for a control system for an industrial robot of the kind mentioned above.